Tectonics of Unusual Crustal Accretion in the Parece Vela Basin - Accretionary Prisms and Convergent Margin Tectonics in the Northwest Pacific Basin - page 159

6˚S

Peridotite
(Spinel Cr# ~0.26)

7˚S

Gabbro

8˚S

9˚S

50 km

Inferred hotspot

10˚S

15˚W 14˚W 13˚W 12˚W 11˚W 10˚W

−6000 −4000 −2000 0 Bathymetry (m)

Fig. 3 Structural image of the Ascension Fracture Zone and the adjacent Mid-Atlantic Ridge
segments illuminated from 315°. Two-hundred meter grid data from Brozena and White ( 1990 )
were utilized (through the Marine Geoscience Data System web site at http://www.marine-geo.
org/ ). Note the corrugated surface and the full axial development (~25 km) of the Ascension OCC.
Location of the dredge hauls that recovered peridotites (and serpentinites) and gabbros during the
R/V Meteor cruise M41 (Schulz et al. 1999 ) are shown. The spinel Cr # (~0.26) of peridotite is
from E. Hellebrand (personal communication, 2002). Some pseudofaults (indicated by dotted
lines ) suggest ridge propagation and jump occurred at this region of the Mid-Atlantic Ridge
(Brozena and White 1990 ). Location of the inferred hotspot is from Brozena ( 1986 )

to those observed in the central PVB (Fig. 3 ): (1) the Ascension OCC extends along
the full length of the short first-order segment (~25 km) between the closely-spaced
North and South Ascension FZs, and (2) the peridotite exposed there is relatively
fertile (spinel Cr# ~0.26; E. Hellebrand, personal communication, 2002) compared
to the ultra-depleted compositions found at the Bouvet FZ (Johnson et al. 1990 )
where a melting anomaly influenced by a hotspot is observed.
The Mid-Atlantic Ridge near the Ascension FZ is thus similar to the central
PVB in that these distinct characteristics of relative magma starvation are observed
in an otherwise robust magmatic budget environment, suggesting a transform sand-
wich effect is working there.